[Graphics] 19 Lighting model (four, Blinn-Phong lighting model)

来源：《UNITY SHADER入门精要》

1、回忆Blinn光照模型

  BlinnIllumination model no use 反射方向、The introduction of the new half vector（bisecoter） $\hat{\mathbf{h}}$,它是通过对视角方向 $\hat{v}$ And the light direction and $\hat{r}$ Want to add more normalization of：
$$\hat{\mathbf{h}}=\frac{\hat{\mathbf{v}}+\mathbf{I}}{|\hat{\mathbf{v}}+\mathbf{I}|}$$
  BlinnModel calculation specular reflection formula is as follows：
$${\mathbf{c}}_{spcular}=\left({\mathbf{c}}_{light}\cdot {\mathbf{m}}_{specular}\right)\max {\left(0,\hat{n}\cdot \hat{h}\right)}^{m_{gloss}}$$

2、Blinn-Phong光照模型的实现

  Here just give a per-pixel implementation.

Shader "Unity Shaders Book/Chapter 6/Blinn-Phong" {
	Properties {
		_Diffuse ("Diffuse", Color) = (1, 1, 1, 1)
		_Specular ("Specular", Color) = (1, 1, 1, 1)
		_Gloss ("Gloss", Range(8.0, 256)) = 20
	}
	SubShader {
		Pass { 
			Tags { "LightMode"="ForwardBase" }
		
			CGPROGRAM
			
			#pragma vertex vert
			#pragma fragment frag
			
			#include "Lighting.cginc"
			
			fixed4 _Diffuse;
			fixed4 _Specular;
			float _Gloss;
			
			struct a2v {
				float4 vertex : POSITION;
				float3 normal : NORMAL;
			};
			
			struct v2f {
				float4 pos : SV_POSITION;
				float3 worldNormal : TEXCOORD0;
				float3 worldPos : TEXCOORD1;
			};
			
			v2f vert(a2v v) {
				v2f o;
				// Transform the vertex from object space to projection space
				o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
				
				// Transform the normal from object space to world space
				o.worldNormal = mul(v.normal, (float3x3)_World2Object);
				
				// Transform the vertex from object spacet to world space
				o.worldPos = mul(_Object2World, v.vertex).xyz;
				
				return o;
			}

  The previous code is the same as before,根据公式,We only change the part of the fragment shader code.
  Remember why 39 Line is the left,Because they both changed the location,To avoid the invert operation.

			fixed4 frag(v2f i) : SV_Target {
				// Get ambient term
				fixed3 ambient = UNITY_LIGHTMODEL_AMBIENT.xyz;
				
				fixed3 worldNormal = normalize(i.worldNormal);
				fixed3 worldLightDir = normalize(_WorldSpaceLightPos0.xyz);
				
				// Compute diffuse term
				fixed3 diffuse = _LightColor0.rgb * _Diffuse.rgb * max(0, dot(worldNormal, worldLightDir));
				
				// Get the view direction in world space
				fixed3 viewDir = normalize(_WorldSpaceCameraPos.xyz - i.worldPos.xyz);
				// Get the half direction in world space
				fixed3 halfDir = normalize(worldLightDir + viewDir);
				// Compute specular term
				fixed3 specular = _LightColor0.rgb * _Specular.rgb * pow(max(0, dot(worldNormal, halfDir)), _Gloss);
				
				return fixed4(ambient + diffuse + specular, 1.0);
			}
			
			ENDCG
		}
	} 
	FallBack "Specular"
}

  And the code is not the same as before is 第14行,采用的是 半程向量（bisecoter）,就是worldLightDir + viewDir ,Then basis, the,然后参与计算.

3、UnityThe built-in help function

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  Some of the function is simple,Don't help function so we can quickly write out,也能够在 UnityCG.cginc 中找到.比如：

  ①WorldSpaceViewDir(float4 v)：

// Computes world space view direction, from object space position
// *Legacy* Please use UnityWorldSpaceViewDir instead
inline float3 WorldSpaceViewDir( in float4 localPos )
{
    float3 worldPos = mul(unity_ObjectToWorld, localPos).xyz;
    return UnityWorldSpaceViewDir(worldPos);
}

  传入的参数是 模型空间 中的位置,Function to turn into world space,再传入unityworldSpaceViewDir(worldPos)函数.
  当然,We can see from the official comments,This is a legacy of the old function,不如直接使用 加了 unity 的版本.

  ②UnityWorldSpaceViewDir( in float3 worldPos )

inline float3 UnityWorldSpaceViewDir( in float3 worldPos )
{
    return _WorldSpaceCameraPos.xyz - worldPos;
}

  There is also carried out 摄像机-顶点位置,这个操作了.其他也没啥了.

  ⑤UnityWroldSpaceLightDir(float4 v)函数
The other three table about LightDir The function of all is the use of the _WorldSpaceLightPos0 The built-in for the direction of the light.Must be prior to rendering it will be correct assignment.

  ⑦ UnityObjectToWorldNormal( in float3 norm )函数

// Transforms normal from object to world space
inline float3 UnityObjectToWorldNormal( in float3 norm )
{
#ifdef UNITY_ASSUME_UNIFORM_SCALING
    return UnityObjectToWorldDir(norm);
#else
    return normalize(mul(norm, (float3x3)unity_WorldToObject));
#endif
}

  法线的 模型空间 到 世界空间 The transformation and other normal vector is different.如果对此有疑问,可以参考《UnityShader入门精要》的4.7节.Can also refer to this series[13 UnityShader入门（四）](13 UnityShader入门（四）.md/## 6、法线变换问题),Through the formula to see why.

  ⑧UnityObjectToWorldDir( in float3 dir )函数：

inline float3 UnityObjectToWorldDir( in float3 dir )
{
    return normalize(mul((float3x3)unity_ObjectToWorld, dir));
}

  把方向矢量从 模型空间 转到 世界空间.